The present invention relates to a method of producing triarylborane. In more detail, it provides a method to produce a highly pure triarylborane stably in high yield by restricting the range of the molar ratio of arylmagnesium halide to boron halide.
It is known that, by reacting an organic magnesium reactant with a boron trihalide such as boron trifluoride ethyl ether complex, a trialkyl- or and arylborane can be obtained (e.g. Inorg. Synth., 15, 1947, P134).
However, it is stated that, if the reaction is conducted at a molar ratio of arylmagnesium halide to boron trihalide even in amount which is just a little in excess of arylmagnesium halide than 3:1, or if the time for dropwise addition of arylmagnesium halide is too fast, then the production of tetraarylborate becomes high resulting in lower yield of triarylborane. This requires severe production conditions and difficult management of the production process.
Moreover, other production methods of triarylborane include a method wherein phenyl bromide is added to metallic magnesium and boron trifluoride ethyl ether complex and then ultrasonic waves are applied (e.g. J. Org. Chem., 51, 1986, P427), a method of synthesizing triarylborane from phenylmagnesium bromide and trialkyl boric acid ester (e.g. U.S. Pat. No. 3,651,146), a method of reacting phenyl lithium with boron trifluoride ethyl ether complex (e.g. A., 563, 1949, P110), etc.
However, the application of ultrasonic waves is undesirable industrially, the use of trialkyl boric acid ester as a source of boron has a drawback of low yield, and the method of using aryl lithium poses a problem of primary production of tetraarylborate.
As described, conventional methods had the problems of low yield and purity of triarylborane and poor reproducibility.
The inventors investigated extensively on the production conditions of triarylborane through the reaction of boron trihalide with arylmagnesium halide. And, the use levels of boron trihalide and arylmagnesium halide were examined at a molar ratio of 1:3 as described in literature for synthesis. As a result, the inventors have known that, if using the synthetic method described in literature (e.g. Inorg. Synth., 15, 1947, P134), then both the yield and the purity of triarylborane are low and the reproducibility is also poor.
The purpose of the invention is to provide a production method which allows one to improve such drawbacks and to obtain triarylborane in higher yield and higher purity.
As a result of diligent studies, the inventors have found that, by reacting boron trihalide with arylmagnesium halide at 3.1 to 3.5 mol of arylmagnesium halide to 1 mol of boron trihalide, and further by removing the ether type solvent from the reaction mixture, thereby solidifying the halogenated magnesium salt produced as a by-product to release triarylborane enclosed in the magnesium salt, triarylborane can be obtained stably in high yield, leading to the completion of the invention.